1. Field of the Invention
The present invention relates to a method of producing semiconductor devices and photomasks therefor, and particularly relates to a method of arrangement of effective chips through shadow printing by means of a photomask or through projection printing by means of a reticle according to the photolithographic technique in a semiconductor device producing method, and photomasks for use in the process of this producing method.
2. Description of the Prior Art
In the process of producing a semiconductor device such as an integrated circuit or the like, conventionally, transfer printing for forming a mask pattern on a semiconductor wafer (hereinafter simply called "a wafer") is an important step as one of photolithographic techniques. As well known, to attain the transfer printing there are two ways, one being shadow printing in which shadowgram projection is formed on a wafer by use of a mask pattern, the other being projection exposure in which an optical system such as a refractive-system lens, a reflective-system spherical mirror, or the like, is interposed between a mask and a wafer. A mask used for the projection exposure in which a refractive-system lens is disposed is called a reticle.
That is, it is shadow printing in which a mask pattern is directly printed on a resist film on a wafer without interposing any optical system between a mask and a wafer. Further, at present, in projection printing, a stepper system of reduced projection transfer is in a main current. The exposure of this system is called stepper exposure. A reticle pattern which is ten times or five times as large as an original pattern is imaged as a chip pattern through a refractive-system reducing lens, and the image is projected on the whole of a wafer through step-and-repeat. An apparatus for performing stepper exposure and the operation thereof are already well known, and therefore the description about them will be omitted here.
First, in or upon completion of the process of the conventional shadow printing using a photomask or projection printing using a reflection-system spherical mirror, chips are arranged on a wafer in such a manner as shown in FIG. 4. That is, normal semiconductor chips 4 each having a desired shape are formed in the most part on the surface of a wafer 3, while semiconductor chips 5 incomplete in appearance as well as in function are formed in a range in the vicinity of an outer circumference of the wafer 3 through the shadowgram of a mask pattern of a photomask (which will be described later). This depends on the desire to use the effective range on the wafer 3 to the maximum.
In this case, the size of the arrangement of the whole of a plurality of chips to be used over a photomask is larger than the diameter of the wafer 3 in FIG. 4 as can be seen by a photomask 1 shown in FIG. 5, so that the chip arrangement is made to have a chip pattern which extends to the outer circumferential range of the wafer 3. This arrangement is used because of simplicity in data processing in producing the photomask 1 including mask patterns 2.
Next, in the producing process through the conventional projection exposure using a refractive-system lens in which a reticle is used, as shown in FIG. 6, an enlarged collective pattern 7, for example, five-times enlarged collective pattern, formed on a reticle 6 is constituted by, for example, four enlarged mask patterns 20. When ordinary stepper exposure is carried out by use of this reticle 6, a mixture pattern in which complete semiconductor chips 4 and incomplete semiconductor chips 5 are mixed is provided on a wafer 3 similarly to the case shown in FIG. 4.
Further, as being apparent from the description of the conventional integrated circuit producing methods in reference to FIGS. 4 through 6, the conventionally used photomask 1 has a pattern which is formed so that the range of the outer periphery of a generally rectangular grid-like pattern 8 having the arrangement of the mask patterns 2 is larger than the outer circumference of the wafer 3 shown in FIG. 4.
Of the incomplete semiconductor chips 5 described above, those having an incomplete appearance as shown in FIG. 7 are detected in an appearance inspection as having insufficient optical reflection, for example, by using an optical microscope or a stereomicroscope, because a chip pattern is printed on a chamfer portion 9 on the wafer 3 as shown by arrows B in FIG. 8. Such an incomplete chip has an appearance of an incomplete a rectangle which has a little cut-away portion in comparison with a regular rectangle so that the appearance is not so good but it has no functional defect as a semiconductor chip. Further, a chip which is incomplete in function has a large cut away portion in comparison with a regular rectangle, so that it has a large defect in function as well as in appearance.
In the conventional semiconductor device producing method as described above and in the photomask to be used in such a method, semiconductor chips formed in the outer circumferential range of a wafer (a semiconductor substrate) include numbers of chips which are incomplete in appearance and/or in function. Accordingly, there have been causes of increase in cost due to increase in the number of steps of selecting chips which are incomplete in appearance and due to prolongation of time taken for the test for removing chips which are incomplete in function.